Frankly it wont matter which flavour you choose (nVidia/ATI) after sales service will be dictated by the "manufacturer" and there are significant differences in the after sales service for almost identical products. I explored this issue when making my Video Card decision:
Damn there is one company/brand in particular that offers a "Limited Lifetime Warranty", just can't remember which and you can find it quite easily on their website but I couldn't get them in Australia.
Imho a single 4870 or GTX260 will give you more performance than you will need and possibly use, unless your going to upgrade to a larger monitor.
I'm allowed to get a BIOSTAR to once I get a job again , damn missus we have a nice chunk of money in the bank but she won't let me till I start working again.